This invention relates to an electric discharge machine for performing an electric discharge machining operation by using a machining solution containing machining particles.
An electric discharge machining technique is well known in the art in which the high temperature energy produced by an electric discharge circuit is utilized to melt an electrically conductive material, such as a metal material, or subject it to surface treatment. In an electric discharge machine operating of this technique, in order to obtain electrical insulation for an electric discharge machining operation, generally the discharge gap is filled with a machining solution which has electrical insulating characteristics. That is, the machining solution is used for electric insulation. In addition, the machining solution is used for removal of the sludge formed during discharge machining, and for cooling the material being machined.
On the other hand, it is known in the art that, when a machining solution is used which contains machining particles about 10 to 40 .mu.m in particle size to a mixing density of is about 20 g/l, a mirror surface machining can be achieved which is heretofore impossible to perform in the case where the confronting area of the electrode and a material to be machined (hereinafter referred to as "a workpiece", when applicable) is large. In addition, it is also known that, with such a machining solution, the physical characteristics, such as corrosion resistance and wear resistance, of the workpiece are improved, depending on the material of the machining particles.
The technique is also well known in the art in which the machining solution is mixed with machining particles such as the above-described machining particles, to form the surface layer of a workpiece. This technique may greatly increase the range of application of electric discharge machining; however, it involves problems to be solved in practical use.
FIG. 14 shows a conventional electric discharge machine using machining particles. In FIG. 14, reference numeral 1 designates an electrode; 2, a material to be machined (or a workpiece); 3, a surface plate on which the workpiece 2 is placed, the surface plate 3 being set on the bottom of a machining vessel 4; 5, a machining solution tank containing a machining solution 7 mixed with machining particles 6; and 8, a machining solution supplying pipe connected between the machining vessel 4 and the machining solution tank 5, the pipe 8 being provided with a pump 9 and a control valve 10.
Further in FIG. 14, reference numeral 11 designates a drain pipe with a drain control valve 29, which is adapted to return the machining solution in the machining vessel 4 into the machining solution tank 5; 12, a coarse machining solution tank for containing a coarse machining solution. The coarse machining solution tank is divided into two vessels; that is, a used machining solution vessel 12a for containing the used machining solution received from the machining vessel 4, and a purified machining solution vessel 12b for containing a purified machining solution which is the machining solution obtained by filtering the used machining solution in the used machining solution vessel 12a.
Further in FIG. 14, reference numeral 14 designates a filter for removing machining particles from the machining solution. The filter 14 is provided in a pipe 15 connected between the used machining solution vessel 12a and the purified machining solution vessel 12b. The used machining solution in the used machining solution vessel 12a is supplied to the filter 14 by a pump 16, where it is filtered. Further in FIG. 14, reference numeral 17 designates a coarse machining solution supplying pipe connected between the purified machining solution vessel 12b and the machining vessel 4, and is provided with a pump 18 and a control valve 19; 20, a used machining solution drain pipe, which is used to deliver the used machining solution in the machining vessel 4 into the used machining solution vessel 12a through a drain control value 21; and 22, the precipitate of machining particles in the machining vessel 4.
The operation of the conventional electric discharge machine thus organized will be described.
In an ordinary coarse machining operation, the pump 18 is operated to supply the purified machining solution from the purified machining solution vessel 12b to the machining vessel 4 through the control valve 19 and the coarse machining solution supplying pipe 17. In the machining vessel 4, electric discharges are induced between the electrode 1 and the workpiece 2 which is fixedly placed on the surface plate 3; that is, the workpiece 2 is subjected to electric discharge machining. In this operation, sludge 13 is formed.
As a result, the machining solution contains the sludge 13. The machining solution in the machining vessel 4 is returned through the used machining solution drain pipe 20 and the drain control valve 21 into the used machining solution vessel 12a. The used machining solution thus returned is sent to the filter 14 by the pump 16, where the sludge 13 is removed from the used machining solution; that is, the latter is purified. The machining solution thus purified is stored in the purified machining solution vessel 12b so as to be used again. The removal of sludge 13 from the used machining solution is carried out all times or intermittently during discharge machining. Thus, a coarse machining operation is performed while the machining solution containing no sludge is being supplied into the machining vessel 4.
After the coarse machining operation, a fine machining operation is carried out, for instance, for surface treatment. In this operation, the coarse machining solution is returned, in its entirety, into the used machining solution vessel 12a, and thereafter the pump 9 is operated to supply the machining solution 7 containing machining particles 6 from the machining solution tank 5 to the gap between the electrode and the workpiece (hereinafter referred to as "an interelectrode gap", when applicable) through the control valve 10.
In an electric discharge machining operation which is carried out while a machining solution containing machining particles is being held between the electrode and the workpiece, the concentration of machining particles in the machining solution must be maintained constant at all times, because it greatly affects the machining stability. For this purpose, during the machining operation, the machining solution 7 is circulated at all times; more specifically, the machining solution 7 is returned through the drain pipe 11 and the drain control valve 19 into the machining solution tank 5 while being supplied into the machining vessel 4 from the machining solution tank 5.
The conventional electric discharge machine is designed as described above. In the machine, the machining solution is circulated so as to supply the machining particles to the interelectrode gap at all times. However, in general, the machining vessel is large in volume, and the flow rate of the machining solution thus circulated is low, and therefore the machining particles in the machining solution are liable to precipitate. Hence, most of the machining particles in the machining solution precipitate on the bottom of the machining vessel as indicated at 22.
After a finish machining operation, the machining solution containing machining particles is returned into the machining solution tank; however, a larger part of the precipitate of machining particles remains in the machining vessel as it is. Hence, the next coarse machining operation will suffer from the following difficulty: That is, while the coarse machining solution supplied into the machining vessel is being circulated, the machining particles flow into the used machining solution vessel 12a, thus being removed by the filter 16. Accordingly, the concentration of machining particles in the machining solution is decreased, which may result in the reduction of surface roughness, in the occurrence of arcs, or in the abnormal consumption of the electrode.
This difficulty may be eliminated by the following method: During discharge machining, the operator agitates the machining solution in the machining vessel to minimize the precipitation of machining particles, and after the machining operation, the operator removes the precipitate of machining particles from the machining vessel into the machining solution tank 5 by himself, or supplies machining particles additionally into the machining solution tank. However, such methods obstruct automation of the electric discharge machine, and involves problems to be solved in economical operation and in running cost.
A device for agitating the machining solution during discharge machining has been proposed in the art (Japanese Patent Application (OPI) No. 144198/1975 (the term "OPI" as used herein means an "unexamined published application")). In the device, nozzles are arranged on a cylindrical member set around the electrode, so to cause the machining solution to flow like a tornado about the electrode. The device is advantageous in that the sludge can be removed from the interelectrode gap; however, it is still disadvantageous in that, in the case where a machining solution containing machining particles is used, the machining particles will precipitate outside the cylindrical member, so that the concentration of machining particles in the machining solution is decreased as in the above-described case.